Anterior cervical plate

ABSTRACT

An anterior cervical plate system is provided. The cervical plate includes a retention ring with a deflectable flange that is upwardly spaced from the top surface of the ring and configured to prevent an inserted bone fastener from backing out of the plate. The plate includes a locking pin having a camming surface and a blocking surface. When the camming surface is moved into position adjacent to the flange, the flange is free to flex out of the way of a bone screw being inserted into or removed from the plate. When the blocking surface is positioned adjacent to the flange, outward deflection of the flange is prevented to retain the bone screw inside the plate. The locking pin is rotated through a camming surface to bring a blocking surface against the flange deflecting the flange onto the head of the bone screw.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and benefit of U.S. patentapplication Ser. No. 14/158,995 entitled “Anterior cervical plate” filedon Jan. 20, 2014 which is a continuation of U.S. patent application Ser.No. 13/185,641, filed on Jul. 19, 2011 now U.S. Pat. No. 8,668,723,issued on Mar. 11, 2014 entitled “Anterior cervical plate” all of whichare incorporated herein by reference in its entireties.

FIELD

This invention relates to bone fixation plates and, more particularly,to fixation plates for the cervical spine that resist the back out ofassociated bone fasteners.

BACKGROUND

Anterior cervical plates are used for a variety of conditions toimmobilize, stabilize or align cervical vertebrae. For example, aftercervical spinal fusion surgery, cervical plates are used to add strengthand rigidity to the adjoined vertebrae. Also, cervical plates securevertebrae together where an intervening vertebra has been removed orreplaced. In other cases, cervical plates are used to correctinstability in the cervical spine caused by trauma, tumors, advanceddegenerative discs, infection or congenital or acquired deformities.

A typical cervical plate includes an elongated rectangular plate thatspans the distance between two or more vertebrae. The plate is curved tomatch the natural curvature of the spine at the location to which it isattached and bone screws are used to fasten the plate to the vertebralbodies. A pair of apertures is formed at one end of the plate forpassing bone screws through and into a first vertebral body to securethe first end of the plate to the first vertebral body. A second pair ofapertures is formed at the other end of the plate for passing bonescrews through and into a second vertebral body to secure the second endof the plate to the second vertebral body. Thereby, the plate bridgestwo vertebral bodies. More vertebrae may be connected with a longerplate and a corresponding increased number of bone screw apertures andbone screws inserted therethrough at the intervening vertebral levels.

The cervical spine can be surgically approached anteriorly orposteriorly. In anterior cervical fusion surgery, an incision is madeand the spine is approached from the front of the patient. The carotidsheath, muscles, trachea and esophagus are moved laterally to expose thecervical spine. Holes are drilled into the vertebral bodies orself-tapping screws are employed. The cervical plate is properly alignedon the vertebrae for the receipt of mounting screws and the plate iscarefully and firmly attached. Sometimes fusion is accompanied by adiscectomy in which a herniated disc is removed and a graft device isplaced between the vertebral bodies to assist in fusion across levels.The plate may also include a window formed generally at a locationbetween the two pairs of screw apertures through which bone growthprogress may be observed. With the plate in position, the vertebrae areheld by the plate in desired spatial relationships and orientationsrelative to each other, pressure is removed from the nerve roots andpain caused by the herniated disc or other condition is relieved.

Over time, the interface between the screws and the bone may presentsome problems of stability. Due to the anatomical structure of thecervical spine and the extreme anatomical forces that are brought tobear on the skeleton and transmitted to the cervical spine, the screwssecuring the plate to the spine may vibrate or toggle out of position.Also, the degeneration of vertebral bone quality may result in thescrews loosening or becoming dislodged. As a result, bone screwssecuring the plate to the spine may move or back out of the vertebralbody and plate. Due to the relative location to the esophagus and otherconnective tissue, if the bone screw securing the plate to the cervicalspine backs out, the bone screw could impinge on the adjacent tissue andincrease pain. Also, loosened screws may result instability of the jointand lead to increased pain for the patient.

Therefore, there is a need to provide a new and improved anteriorcervical plate that resists fasteners, such as bone screws, from backingout of the plate and also from being loosened with respect to the platebefore migrating out. Not only an improved and effective fastenerretaining mechanism is required, but also, its design cannot add unduebulk to the plate. The anterior cervical plate must have a low profiledue to the proximity of the implant site to the esophagus and othersensitive surrounding tissue. It is also preferable to keep the plate asnarrow as possible to reduce the chances that the lateral edges rise offfrom the underlying vertebral body and cause pain where the curvature ofthe plate does not exactly match the patient's anatomy. Furthermore,there is a need for the anterior cervical plate to withstand anatomicalforces and be easily implanted. Also, the screw retaining mechanism mustbe easily activated by the surgeon. This invention, as described in thedetailed description, sets forth an improved anterior cervical platewith anti-back out protection that meets these needs.

SUMMARY

According to one aspect of the invention, a bone plate is provided. Thebone plate includes at least one through hole configured to receive abone screw for attaching the plate to bone. The bone plate includes aretention ring disposed inside the at least one through hole. Theretention ring includes a central aperture having a entry opening at atop surface. The retention ring includes a resiliently deflectableretention flange upwardly spaced from the top surface of the retentionring. The retention flange has a portion extending radially inwardlyabove the entry opening when in a normal undeflected position. The plateincludes a bone screw having a head portion connected to a shankportion. The bone screw is configured for insertion into the throughhole and into the central aperture of the retention ring such that atleast a portion of the head portion is positioned distally of theretention flange. The plate includes a locking pin being movablyconnected with respect to the plate and mechanically coupled to at leastone adjacent retention flange. The locking pin has at least one blockingsurface. The bone plate includes a locked position in which the blockingsurface of the locking pin is moved into a position adjacent to theretention flange to prevent outward deflection of the retention flangeand thereby maintain the retention flange in the pathway of the entryopening and above the bone screw to prevent the bone screw from backingout of the through hole.

According to another aspect of the invention, a bone plate is provided.The bone plate includes a plate having at least one through holeconfigured to receive a bone screw for attaching the plate to bone. Aretention ring is disposed inside the through hole. The retention ringincludes a central aperture having a entry opening at a top surface andan exit opening at a bottom surface. The central aperture defines acentral axis. The retention ring further includes an inner surfaceinterconnected to an outer surface with each extending between the topsurface and the bottom surface. The retention ring further includes aplurality of slots spaced circumferentially around the retention ring.The slots extend from the top surface towards the bottom surface to formdeflectable tabs between the slots. The bone plate includes a bone screwhaving a head portion connected to a shank portion. The bone screw isconfigured for insertion into the through hole and into the centralaperture of the retention ring. When the retention ring is inserted intothe at least one through hole, the retention ring is configured suchthat the tabs are deflected inwardly towards the central axis such thatthe tabs cover at least a portion of the head portion of the bone screwthat is inserted into the central aperture of the retention ring.

According to another aspect of the invention, a bone plate is provided.The plate has at least one through hole configured to receive a bonescrew for attaching the plate to bone. A retention ring is disposedinside the through hole. The retention ring includes a central aperturehaving an entry opening at a top surface and an exit opening at a bottomsurface. The central aperture defines a central axis. The retention ringfurther includes an inner surface and an outer surface extending betweenthe top surface and the bottom surface. The retention ring also includesa resiliently deflectable retention flange upwardly spaced from the topsurface of the retention ring. The retention flange has a portion thatextends radially inwardly above the entry opening when in a normalundeflected position. The bone plate includes a bone screw having a headportion connected to a shank portion. The bone screw is configured forinsertion into the through hole and into the central aperture of theretention ring such that at least a portion of the head portion ispositioned distally of the retention flange. The bone plate alsoincludes a locking pin connected to the plate and mechanically coupledto at least one adjacent retention flange. The locking pin has at leastone blocking surface and at least one camming surface. The locking pinis movable to selectively position the blocking surface and cammingsurface adjacent to the retention flange. When positioned adjacent tothe retention flange, the blocking surface is configured to preventoutward deflection of the retention flange to prevent the bone screwfrom backing out of the through hole. When positioned adjacent to theretention flange, the camming surface is configured to allow clearancefor the retention flange to deflect outwardly to permit the bone screwto pass into and out of the retention ring.

According to another aspect of the invention, a method of using a spinalplate system is provided. The method includes the step of attaching aplate to bone wherein the plate comprises a plurality of through holesand a retention ring disposed in at least one through hole. Theretention ring has a central aperture and a resiliently deflectableretention flange extending toward a central axis of the retention ring.The plate further includes at least one locking pin located adjacent tothe at least one through hole and mechanically coupled to an associatedadjacent retention flange. The method further including the steps ofinserting a bone screw having a head connected to a threaded shank intoeach central aperture and through hole and inserting the bone screw intovertebral bone. Insertion of the bone screw is terminated when at leasta portion of the head of the bone screw is located distally of theretention flange and the retention flange covers at least a portion ofthe bone screw or until a click is heard or felt. The method furtherincludes the steps of rotating the at least one locking pin andterminating rotation of the at least one locking pin when the retentionflange is locked in position or until a click is heard or felt.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top perspective view of an anterior cervical plate systemaccording to the present invention.

FIG. 2 is a top perspective exploded view of an anterior cervical platesystem according to the present invention.

FIG. 3 is a top planar view of an anterior cervical plate systemaccording to the present invention.

FIG. 4 is a side elevation view of an anterior cervical plate systemaccording to the present invention.

FIG. 5 is an end elevation view of an anterior cervical plate systemaccording to the present invention.

FIG. 6 is a top perspective view of a plate according to the presentinvention.

FIG. 7 is a top planar view of a plate according to the presentinvention.

FIG. 8 is a cross-sectional view taken along line A-A of FIG. 7 of aplate according to the present invention.

FIG. 9 is a cross-sectional view taken along line B-B of FIG. 7 of aplate according to the present invention.

FIG. 10 is a side elevation view of a plate according to the presentinvention.

FIG. 11 is a bottom planar view of a plate according to the presentinvention.

FIG. 12 is a top perspective view of a bone fastener according to thepresent invention.

FIG. 13 is a side elevation view of a bone fastener according to thepresent invention.

FIG. 14 is a top planar view of a bone fastener according to the presentinvention.

FIG. 15 is a top perspective view of a retention ring according to thepresent invention.

FIG. 16 is a top planar view of a retention ring according to thepresent invention.

FIG. 17 is a side elevation view of a retention ring according to thepresent invention.

FIG. 18 is a back end elevation view of a retention ring according tothe present invention.

FIG. 19 is a front end elevation view of a retention ring according tothe present invention.

FIG. 20 is a top perspective view of a locking pin according to thepresent invention.

FIG. 21 is a top planar view of a locking pin according to the presentinvention.

FIG. 22 is a side elevation view of a locking pin according to thepresent invention.

FIG. 23 is a side elevation view taken 90 degrees to FIG. 22 of alocking pin according to the present invention.

FIG. 24 is a top sectional view of an anterior cervical plate systemwith a locking pin in an unlocked position according to the presentinvention.

FIG. 25 is a cross-sectional view taken along line C-C of FIG. 3 of ananterior cervical plate system with the locking pin in an unlockedposition according to the present invention.

FIG. 26 is a top sectional view of an anterior cervical plate systemwith a locking pin in a locked position according to the presentinvention.

FIG. 27 is a cross-sectional view taken along line B-B of FIG. 3 of ananterior cervical plate system with the locking pin in a locked positionaccording to the present invention.

FIG. 28 is a top perspective sectional view of the anterior cervicalplate system with a locking pin in a locked position according to thepresent invention.

FIG. 29 is a flow chart illustrating a method of securing the cervicalplate system according to the present invention to the cervical spine ofa patient.

DETAILED DESCRIPTION

FIGS. 1-5 depict a cervical plate system 10 according to one variationof the invention that may be used to stabilize or fuse vertebral bodiesin the cervical or other region of the spine. The anterior cervicalplate system 10 that is shown in FIGS. 1-5 is a two-level bone fixationplate that is configured to span across and fixate three vertebrae ofthe cervical spine although the cervical plate system 10 may be a singlelevel or any multilevel anterior cervical plate spanning two or morevertebral bodies. The anterior cervical plate system 10 comprises aplate 12 having fasteners 14 passed through retention rings 16 lockedinto place with locking pins 18.

Turning now to FIGS. 6-11, the plate 12 will now be described in greaterdetail. The plate 12 includes an upper surface 20 or anterior surfacethat faces the patient's soft tissue and esophagus when installed and alower surface 22 or posterior surface facing the vertebral bodies to beimmobilized. The upper surface 20 and lower surface 22 areinterconnected by curved side walls and end walls to form a generallyrectangular shape that is symmetrical about a midline. As best seen inFIGS. 8 and 9, the gently curved structure of the rectangular plate 12complements the natural curved structure of the vertebral bodies andlordotic curvature of the cervical spine. The corners of the plate arerounded to reduce or eliminate irritation of the esophagus and thesurrounding tissue. The plate 12 is sized and shaped for use on ananterior aspect of the cervical spine although one skilled in the artmay use the device in other regions of the spine and other skeletalfixations. The plate 12, which resides atop the vertebral bodies, has alow profile as seen in FIG. 10 so as to minimally impinge on adjacenttissues.

The plate 12 and other components of the cervical plate system 10 aremade from suitable biocompatible material such as stainless steel,titanium and or any other metal or metal alloy. One or more componentsmay be made of non-metal materials including but not limited to polymer,carbon reinforced polyetheretherketone (PEEK) or one or morebiocompatible ceramics. The plate 12 may be additionally configured topromote bone ingrowth to the plate such as a portion of the plate beingmade of porous material or being roughened by mechanical blasting orplasma spraying with metal particles of one or more sizes. The plate 12may also be coated with bio-active material, therapeutic agents forenhancing bone fusion and ingrowth, bone morphogenic proteins, growthfactors and the like.

Still referencing FIGS. 6-11, the plate 12 includes a plurality ofthrough holes 24 extending through the cervical plate 12 from the uppersurface 20 and through the lower surface 22. The holes 24 are configuredto receive bone fasteners 14 passed there through. Each hole 24 includesa head-receiving portion 26 near the upper surface 20 connected to asmaller shank-receiving portion 28 near the lower surface 22 to,thereby, in one variation, provide a seat for the head portion of thefastener 14 at a ledge 29 formed at the intersection of thehead-receiving portion 26 and shank-receiving portion 28. Thehead-receiving portion 26 is recessed from the top surface 20 as bestseen in FIGS. 8 and 9 such that the head of the fastener 14 does notprotrude beyond the upper surface 20 of the plate 12 in order tomaintain a low profile for the plate 12. Each through hole 24 has alarger exit opening 30 at the lower surface 22 to allow room for theangulation of inserted fasteners 14. In one variation, thehead-receiving portion 26 also provides a receiving well for theretention ring 16. Accordingly, the head-receiving portion 26 is shapedto complement the shape of the retention ring 16. For example, thehead-receiving portion 26 forms a part-spherical seat or curved surfaceconfigured for a complimentary part-spherical or curved outer surface ofthe retention ring 16. In one variation, the size of the through hole 24is configured such that the head-receiving portion 26 andshank-receiving portion 28 are both large enough to allow a bonefastener 14 to pass all the way through the plate without the presenceof a retention ring 16 and wherein the presence of the retention ring 16in the through hole 24 reduces the size of the through hole 24 such thatthe head portion of the fastener 14 is not allowed to pass through theretention ring 16. In another variation, the shank-receiving portion 28of the through hole 24 is smaller than the head-receiving portion 26without the presence of a retention ring 16 such that the head portionof a fastener 14 is not allowed to pass into the shank-receiving portion28 of the through hole 24 and wherein the presence of the retention ring16 further reduces the opening at the head-receiving portion 26 of thethrough hole 24. A notch 32 is formed in at least one of thehead-receiving portion 26 and shank-receiving portion 28. The notch 32prevents the retaining ring 16 from rotating or moving out of place withrespect to the plate 12. In one variation, the notch 32 creates spacewithin which the neck and flange of the retention ring 16 may move andflex as will be discussed in greater detail below. An undercut (notshown) in the through hole 24 such as in the location of thehead-receiving portion 26 may be formed and configured to mate with theretention ring 16 to, thereby, couple the retention ring 16 to thethrough hole 24 as the retention ring 16 is compressed and then insertedinto or under the undercut. In another variation, the through hole 24 isslightly elliptical in shape that matches a slightly ellipticalretention ring 16 which can be inserted in the conforming direction andthen rotated into a non-conforming orientation to be retained within thethrough hole 24 by compression fit engagement therewith.

FIGS. 6-12 depict a plate 12 having three sets or three pairs offastener through holes 24 spaced-apart along the plate centerline fordriving fasteners 14 into and stabilizing three vertebral bodies forcreating a two-level construct. Each set of fastener through holes 24includes two holes 24 spaced apart from each other along the centerlineof the anterior cervical plate 12. Each set or pair of through holes 24is adapted for receiving two fasteners 14 to be driven into a singlevertebral body. As best seen in FIG. 8, the longitudinal axes 25 of apair of through holes 24 diverge relative to each other such that a pairof fasteners 14 placed therein diverge slightly relative to each otherat a desired angled as best seen in FIG. 5.

The plate 12 further includes a recess 34 located between the throughholes 24 of each pair of through holes 24. The recess 34 is configuredfor receiving a locking pin 18 such that the locking pin 18 does notprotrude from the upper surface 20 of the plate 12 in order to maintainthe desired low profile. A locking pin aperture 36 is formed in therecess 34 at the centerline for coupling the locking pin 18 to the plate12. The plate 12 also includes two larger openings 38 located betweeneach pair of through holes 24 that effectively reduce the overall weightof the plate 12 and provide a visualization pathway to monitor bonegraft progress between the vertebral bodies.

With particular reference to FIGS. 12-14, an exemplary orthopedicfastener 14 that is preferably used with the cervical plate system 10 ofthe present invention is a bone screw 14. The bone screw 14 includes ascrew head 40, neck 42 and threaded shank 44. The head 40 includes aledge 47 which is a flat surface along at least a portion of theperimeter of screw head 40. The head 40 includes an instrument recess 46for receiving a complementary tip of a surgical tool. A substantiallyhexagonal, daisy-shaped recess 46 is shown in FIGS. 12-14, however, therecess 46 can be of any shape that allows a surgical tool to drive thebone screws 14 into the vertebral column. The head 40 of the bone screw14 corresponds to the shape of the inside of the associated retentionring 16 or, in an alternative variation, to the shape of thehead-receiving portion 26 of the through hole 24. Various bone screws 14may be employed including ones capable of polyaxial, variable angle orfixed angled orientation with respect to the plate 12 with or withoutthe ability to be locked down at a desired angle or orientation withrespect to the plate 12. The bone screws 14 are preferably self-tapping,however, other screws requiring holes to be drilled or pre-tapped canalso be employed.

Turning now to FIGS. 15-19, the retention ring 16 will now be discussed.The retention ring 16 has a circular or slightly elliptical profile anda central aperture 48 having a central axis and defining an entryopening at a top surface of the retention ring 16 and an exit opening atthe bottom surface of the retention ring 16. The retention ring 16 isconfigured for insertion into and being received inside a through hole24 of the plate 12. The central aperture 48 has a width greater than thewidth of the threaded shank 44 of a bone screw 14 where the width is themajor diameter of the threads or, in an alternative variation, greaterthan the width of the minor diameter of the threads. However, the widthof the central aperture 48 is not larger than the width of the head of afastener 14 such that the head 40 of the fastener is not allowed tocompletely pass through the central aperture 48. The head 40 is receivedat the inner surface 49 of the retention ring 16, which is sized toprevent lateral movement of the fastener 14 and shaped to complement theshape of the screw head 40. For example, the inner surface 49 of theretention ring 16 forms a part-spherical or curved seat configured formulti-angular articulation with a complimentary part-spherical or curvedsurface of the screw head 40 of fastener 14. Of course, thecomplementary surfaces of the retention ring 16 and screw head 40 arenot limited to being part-spherical or curved but may be of any shape.The inner surface 49 of the retention ring 16 generally slopes upwardlyfrom the bottom surface to the top surface of the retention ring 16.

A plurality of slots 50 are formed in the ring 16. The slots 50 extendapproximately halfway from the top surface of the ring 16 toward thebottom surface of the ring 16. The slots 50 form a plurality ofcircumferential tabs at the upper surface around the central aperture49. These slots 50 weaken the upper portion of the ring 16 such that theretention ring 16 is slightly compressible. The compressibility of thering 16 affords advantages for increasing the purchase of the screw head40 to the plate 12 in addition to allowing the retention ring 16 to beinserted and retained in the plate 12. The retention ring 16 is easilyinserted into the head-receiving portion 26 which serves as a well forthe retention ring 16. In one variation, the retention ring 16 mayinclude an externally protruding annular retention lip (not shown) thatwould snap into an undercut or the like formed in the plate 12 andconfigured for connecting the retention ring 16 to the plate 12. Inanother variation, the tabs are deflected slightly inwardly towards thecentral axis upon insertion into a through hole 24 wherein the inwardlydeflected tabs advantageously create an undercut for retaining the screwhead 40 firmly inside the plate 12 through hole 24. The slightlyinwardly deflected tabs serve as fingers grasping and contactingslightly over and around at least a portion of the screw head 40.

Still referencing FIGS. 15-19, extending upwardly from the top surfaceof the ring 16 is a retention flange 54 connected to a neck 52. Theretention flange 54 includes a portion that projects inwardly towardsthe center of the ring 16 and is selectively imposed into the pathway ofa fastener 14. In particular, the retention flange 54 extends into theprojection of the entry opening of the central aperture 48 such that afastener 14 moving into and through the central aperture 48 wouldencounter resistance from the retention flange 54. The retention flange54 includes a scalloped portion 56 that serves as a ramp for guiding abone fastener 14 and for receiving a lateral force component exerted bythe fastener 14 onto the retention flange 54 as a bone fastener 14passes into the central aperture 48 and into the through hole 24. As aresult of the lateral force component exerted upon the retention flange54 by an entering fastener 14, the neck 52 is adapted to flex causingthe retention flange 54 to deflect slightly outwardly to allow a bonefastener 14 to continue to pass through the central aperture 48. Afterthe fastener 14 passes beyond the retention flange 54, the retentionflange 54 is configured to snap back to its normal un-flexed orpartially flexed position such that at least a portion of the retentionflange 54 projects back inwardly toward the center of the ring 16 andinto the pathway of the fastener 14 thereby, forming a stop thatprevents the fastener 14 from backing out. The undersurface 55 of theretention flange 54 overlays the head 40 of the fastener 14 eithertouching the head 40 of the fastener 14 or laying spaced apart from thehead 40 of the fastener 14 and, thereby, preventing the fastener 14 frombacking out. The snapping back of the retention flange 54 as a screwledge 47 passes beyond the retention flange 54 advantageously providesthe surgeon with an audible and/or haptic signal or feedback that thescrew 14 has been advanced far enough and the surgeon can stop drivingor advancing the screw 14 into the vertebral bone. As a result, theaudible or haptic click notifies the surgeon that the screw 14 is inposition and prevents the surgeon from applying too much torque. As bestseen in FIG. 16, the inner facing surface 57 of the retention flange 54is curved to match the curvature of the fastener 14 and configured tocover a portion of the ledge 47 along the perimeter of the screw head40. In another variation, the retention ring 16 is integrally formedwith the plate 12 such that a neck 52 and retention flange 54 projectfrom a surface of the plate 12.

Turning to FIGS. 20-23, the locking pin 18 will now be discussed. Thelocking pin 18 includes a main body 58 connected to a post 60. The post60 extends from the bottom surface 72 of the main body 58 along thelongitudinal axis of the locking pin 18. The post 60 is configured to beinserted into the locking pin aperture 36 of the plate 12 and connectedto the plate 12 such that the locking pin 18 can rotate relative to theplate 12 about the longitudinal axis. The locking pin 18 may include anadditional coupling element (not shown) for coupling the locking pin 18to the cervical plate 12 in a manner that maintains the locking pin 18rotatably coupled to the plate 12. Of course, the locking pin 18 is notlimited to rotational movement with respect to the plate 12 and can bedesigned for linear movement with respect to the plate 12 for example.Whereas the post 60 is inserted into the plate 12, the main body 58 ofthe locking pin 18 resides above the upper surface 20 of the plate 12 inthe location of the recess 34 next to a through hole 24 or in anothervariation as shown in the figures in the location of the recess 34between two adjacent through holes 24 such that the main body 58 of thelocking pin 18 does not extend beyond the outer profile of the plate 12maintaining the smooth low profile of the plate 12. The locking pin 18is shown in the figures to have a circular top profile, however, theinvention is not so limited and the locking pin 18 may be any operableshape.

The locking pin 18 is means for locking or unlocking the retentionflange 54 of the retention ring 16. The locking pin 18 includes acamming surface 66 and a blocking surface 68 formed in the main body 58.The locking pin 18 is positioned next to the retention ring 16 such thatthe camming surface 66 and blocking surface 68 in turn contact at leasta portion of the neck 52 and/or at least a portion of the retentionflange 54 of the stationary retention flange 54. The camming surface 66is adjacent to the blocking surface 68 on the main body 58 andconfigured such that, with rotation of the locking pin 18, at least aportion of the neck 52 and/or at least a portion of the retention flange54 that is in contact with a the camming surface 66 is led into orcammed into being in contact with the blocking surface 68 to lock theretention flange 54 in position. A stop (not shown) may be formed at theend of the blocking surface 68 to prevent further rotation of thelocking pin 18 in the same direction. In one variation without a stop,continued rotation of the locking pin 18, at least a portion of the neck52 and/or at least a portion of the retention flange 54 that is incontact with the blocking surface 66 remains intact with the blockingsurface 66 throughout the rotation of the locking pin 18 until reachingand contacting the same camming surface 66. Such a variation of thelocking pin 18 is employed adjacent to one retention ring 16 and isdesigned to lock one retention flange 54. Of course, rotation of thelocking pin 18 in the opposite direction will lead or cam the at least aportion of the neck 52 and/or the at least a portion of the retentionflange 54 that is in contact with a the blocking surface 68 into beingin contact with the camming surface 66 to unlock the retention flange54.

While still referencing FIGS. 20-23 and with particular reference toFIGS. 24-27, in the variation of the locking pin 18 that is shown inFIGS. 20-23, the locking pin 18 is located between two adjacentretention rings 16 a, 16 b and the locking pin 18 is configured with twooppositely disposed camming surfaces 66 a, 66 b and two oppositelydisposed blocking surfaces 68 a, 68 b for locking or unlocking theretention flanges 54 a, 54 b of two adjacent retention rings 16 a, 16 bsimultaneously. In such a variation, a first camming surface 66 a isadjacent to a first blocking surface 68 a and the second camming surface66 b is adjacent to the second blocking surface 68 b and configured suchthat, with rotation of the locking pin 18, at least a portion of theneck 52 and/or at least a portion of a first retention flange 54 a of afirst retention ring 16 a that is in contact with the first cammingsurface 66 a is led or cammed into being in contact with the firstblocking surface 68 a and at least a portion of the neck 52 and/or atleast a portion of a second retention flange 54 b of a second retentionring 16 b that is in contact with the second camming surface 66 b is ledor cammed into being in contact with the second blocking surface 68 b tolock the retention flanges 54 a, 54 b of the first and second retentionrings 16 a, 16 b simultaneously. Stops may be formed at the end of eachblocking surface 68 a, 68 b to prevent further rotation in the samedirection. Rotation of the locking pin 18 in the opposite direction willresult in the retention flanges 54 a, 54 b that are in contact with theblocking surfaces 68 a, 68 b, respectively, being in contact with thecamming surfaces 66 a, 66 b, respectively, to simultaneously unlock theretention flanges 54 a, 54 b, respectively. In one variation, thelocking pin need only be rotated by 90 degrees to move from an unlockedposition to a locked position or from a locked position to an unlockedposition.

Turning back to FIGS. 20-23, the main body 58 further includes a slit 62configured to receive an instrument, such as a screwdriver, to turn thelocking pin 18 with respect to the plate 12. Although a slit 62 that isconfigured to match a flat screwdriver is shown in FIGS. 20-23, a recesshaving any shape that is complementary to the instrument employed toactivate, move or rotate the locking pin 18 may be used.

Still referencing FIGS. 20-23 and further referencing FIGS. 24-27, themain body 58 includes two scallops 64 a, 64 b located on either side ofthe slit 62. A camming surface 66 is formed in the location of eachscallop 64. In one variation, the locking pin 18 is configured to bedisposed in the cervical plate 12 adjacent to a pair of through holes 24and configured to simultaneously lock two adjacent retention rings 16residing in the pair of adjacent through holes 24 as discussed above. Insuch a variation, the locking pin 18 includes a first camming surface 66a at the first scallop 64 a and a second camming surface 66 b at thesecond scallop 64 b configured to simultaneously cam against twoadjacent retention rings 16 a, 16 b.

Generally, the camming surface 66 comprises a gently curved, or angled,wedge-like surface having a thickness that varies along the top surface70 of the wedge. The bottom of the wedge coincides with the bottomsurface 72 of the main body 58 and is substantially planar. The cammingsurface 66 a of a wedge at the first side varies along the outerperimeter and is the thinnest at a location 90 degrees to the slit 62 asbest seen in FIG. 23. The wedge increases in thickness along theperimeter in a direction toward 0 degrees and 180 degrees as can beclearly seen in FIG. 22. The thickness of the camming surface 66 a alsoincreases toward the longitudinal axes of the locking pin 18. Thecamming surface 66 b of a wedge at the second side is the thinnest at alocation 270 degrees to the slit 62. The wedge increases in thickness ina direction towards 180 degrees and 360 degrees as can also be seen inFIG. 23. The thickness of the camming surface 66 b also increases towardthe longitudinal axes of the locking pin 18. The camming surface 66 issized and configured such that, upon assembly of the plate system 10, atleast a portion of the camming surface 66 is positioned underneath aportion of the retention flange 54 that extends outwardly beyond theperimeter of the retention ring 16 as best seen in FIGS. 25 and 27. Inone variation, the camming surface 66 is angled or curved to camunderneath or against the retention flange 54 and/or neck 52. In thevariation in which the locking pin 18 includes two camming surfaces 66a, 66 b, at least a portion of each camming surface 66 a, 66 b ispositioned underneath each retention flange 54 a, 54 b of the pair witheach camming surface 66 a, 66 b being angled or curved to cam underneathor against the retention flanges 54 a, 54 b and/or necks 52 a, 52 b asbest seen in FIGS. 25 and 27.

From a starting position at 90 degrees from the slit 62 at a locationconfigured to contact the retention flange 54, the portion of thecamming surface 66 that is in contact with the retention ring 16 is thethinnest and, with rotation of the locking pin 16 in a clockwise orcounterclockwise direction, the portion of the camming surface 66 thatis in contact with the retention ring 16 increases in thickness. In onevariation, the camming surface 66 does not contact the retention ring at90 degrees. In the variation in which the locking pin 18 includes twocamming surfaces 66 a, 66 b, the portions of the camming surfaces 66 a,66 b at a location 90 and 270 degrees, respectively, from the slit 62are the thinnest and, with rotation of the locking pin 16 in a clockwiseor counterclockwise direction, the portions of the camming surfaces 66a, 66 b that are in contact with the retention rings 16 a, 16 b,respectively, increase in thickness. In one variation, the cammingsurfaces 66 a, 66 b do not contact the retention rings 16 a, 16 b at 90and 270 degrees. As the locking pin 18 rotates through 90 degrees, thethickness of the camming surface 66 increases to contact with theretention ring 16 at the neck 52 and/or at the retention flange 54.Continued rotation of the locking pin 18 results in the camming surface66 camming against the neck 52 and/or retention flange 54 and moving ordeflecting the retention flange 54 inwardly towards the center of thecentral aperture 48 or towards the central axis of the central aperture48. In one variation, the camming surface 66 contacts or cams against atleast a portion of the undersurface 55 of the retention flange 54 thatlies beyond the outside perimeter of the retention ring 16. Withcontinued rotation of the locking pin 18, the camming surface 66 exertsa force on the retention flange 54 flexing the neck 52 inwardly anddeflecting the flange 54 inwardly toward the center of the centralaperture 48. At the termination of the 90-degree rotation of the lockingpin 18, the blocking surface 68 is adjacent to or abuts at least aportion of the retention flange 54 and/or neck 52. In another variation,at the termination of the 90 degree rotation of the locking pin 18, theneck 52 of the retention ring 16 comes into alignment with the slit 62of the locking pin 18 and is no longer biased or pushed inwardly by thecamming surface 66 beyond the unbiased position of the retention flange54 and neck 52 and therefore, the retention flange 54 is allowed to snapback into its normal position at which point the outer surface 53 of theretention flange 54 is adjacent to or abuts the blocking surface 68 andthe blocking surface 68 thereby creating a stop for the retention flange54 that does not allow the retention flange 54 or neck 52 to flexoutwardly and out of the path of fastener 14 that may be backing out. Inone variation, the retention flange 54 is deflected inwardly beyond anormal undeflected position to contact and cover at least a portion ofthe screw head 40. In another variation, the retention flange 54 isdeflected inwardly beyond a normal undeflected position to contact,cover and additionally exert a force on at least a portion of the screwhead 40 to prevent an inserted bone screw 14 from backing out. Theretention flange 54 is maintained in a locked position with the lockingpin turned through at least 90 degrees of rotation. In one variation,the snapping back of the retention flange 54 against the blockingsurface 68 or into a normal unbiased, undeflected positionadvantageously provides the surgeon with haptic and/or audible feedbacknotifying the surgeon that sufficient rotation of the locking pin 18 isachieved and that a locked relationship of the locking pin 18 with theretention ring 16 is established. Rotation of the locking pin 16 in theopposite direction or continued rotation in the same direction willresult in the camming surface 66 contacting the retention flange 54freeing it to flex out of the path of a fastener 14 to achieve anunlocked relationship of the locking pin 18 and retention ring 16 asshown in FIGS. 24 and 25.

In one variation, a portion of the undersurface 55 of the retentionflange 54 may have a shape that is complementary to the camming surface66. In another variation, the portion of the undersurface 55 that liesoutside the outer perimeter of the retention ring 16 may be angled orcurved to facilitate the camming of the undersurface 55 against thecamming surface 66. Also, the portion of the undersurface 55 that liesinside the outer perimeter of the retention ring 16 may be configured tocontact and closely cover the bone fastener 14.

The cervical plate system 10 is assembled by first inserting the lockingpins 18 into the locking pin apertures 36 located between each pair ofthrough holes 24. The locking pins 18 are secured to the plate 12 suchthat the locking pins 18 are permitted to move or rotate with respect tothe plate 12. An additional coupling mechanism may be employed toconnect to the post 60 of the locking pin 18 from the lower surface 22of the plate 12. Next, the retention rings 16 are inserted into thethrough holes 24 of the plate 12. As mentioned above, each retentionring 16 is slightly compressible due to the slots 50 formed in the uppersurface of the ring 16. The retention rings 16 are compressed andinserted into the through holes 24 and then allowed to expand in thescrewhead-receiving portion 26 of the through hole 24 being retained inthe through hole 24 by way of a friction fit engagement. Alternatively,as mentioned above, the retention ring 16 may include an annular lipextending radially outwardly and configured to engage with acomplementary shaped undercut formed in the plate 12 to connect theretention ring 16 to the plate 12. In yet another variation, the throughhole 24 is slightly elliptical in shape that matches a slightlyelliptical retention ring 16 which can be inserted in a conformingdirection and then rotated into a non-conforming orientation withrespect to the through hole 24 to be retained within the through hole 24by a compression fit engagement. In another variation, the tabs aredeflected slightly inwardly towards the central axes upon insertion ofthe retention ring 16 into a through hole 24 wherein the inwardlydeflected tabs advantageously create an undercut for retaining the screwhead 40 firmly inside the plate 12 through hole 24. In one variation,the camming surface 66 is sized and configured such that, upon assemblyof the plate system 10, at least a portion of the camming surface 66 ispositioned underneath the retention flange 54 of the retention ring 16.If a locking pin 18 is configured to lock two adjacent retention rings16, then at least a portion of both camming surfaces 66 a, 66 b arepositioned underneath or adjacent to the retention flanges 54 a, 54 b.After the retention rings 16 are inserted into the plate 12, the plate12 is ready to be implanted into the patient.

FIG. 29 is a process flow diagram illustrating a method of performing asurgical procedure employing the cervical plate system 10 of the presentinvention. In step 100, the anterior cervical plate 12 is placed orattached adjacent to a vertebral column. The placement of the plate 12relative to the vertebral bone in a patient may be pre-operativelydetermined based on a pre-operative examination of the patient's spineusing non-invasive imaging techniques known in the art. Any additionalpreparation or work may be done on and around the desired vertebraeprior to positioning the plate 12.

Once the plate 12 is appropriately positioned, it may be necessary toturn the locking pins 18 into an unlocked position or check to make surethat they are in an unlocked position in step 102. Next, bone fasteners14 are inserted into adjacent through holes 24 of the plate 12 in step104 while the adjacent locking pin 18 is in an unlocked position. Toinsert a bone fastener 14, an instrument is inserted into the instrumentrecess 46 of the fastener 12 and the fastener 12 is driven or screwedinto the desired bone in step 106. As each bone fastener 14 passes intoa through hole 24, it encounters the retention flange 54 and deflectsthe retention flange 54 outwardly until the head 40 of the fastener 14or, in particular, the ledge 47 of the screw head 40 has traveled pastthe retention flange 54. At this point, the retention flange 54 snapsback such that the retention flange 54 partially overlays or covers thefastener head 40. In particular, the retention flange 54 will cover oroverlay the fastener ledge 47. The snapping-back of the retention flange54 onto the fastener head 40 advantageously provides an audible clickingsound or clicking feeling to the surgeon signaling that the fastener 14is properly seated and need not be driven further into the vertebralbone in step 108. Without this signal to the surgeon, the surgeon maycontinue to drill the fastener 14 into the bone which may detrimentallyaffect implantation. Further feedback is provided to the surgeon in thatthe locking pin 18 cannot be moved into the locked position until andunless at least a portion of the screw head 40 has been fully insertedsuch that the screw head 40, in particular, the ledge 47 of the screwhead 40 is disposed distally of the retention flange 54. The feedbackmay also be visual as a result of the surgeon observing the position ofthe retention flange 54 relative to the screw 14 making sure that theretention flange 54 overlays a portion of the screw 14. The retentionflange 54 may be colored to enhance visual feedback. Another advantageis that the fastener head 40 is seated in the screwhead-receivingportion 26 of the plate 12 against a slightly compressible retentionring 16. The slightly compressible retention ring 16 advantageouslyincreases purchase of the fastener 14 to the plate 12 by bearing ordampening various anatomical forces imposed onto the fastener 14 insteadof directly transmitting such forces undampened to the plate 12 and fromthe plate 12 to other weaker portions of the vertebral anatomy.Furthermore, the tabs are deflected slightly inwardly towards thecentral axes upon insertion of the retention ring 16 into a through hole24 creating a reduced-diameter entry way or undercut for the screw head40 advantageously covering the screw head 40 in finger-like fashion toretain the screw 14 in place.

Once the fasteners 14 are correctly positioned in the through holes 24,the locking pin 18 is rotated in step 110. To rotate the locking pin 18,an instrument is inserted into the slit 62 or recess of the locking pin18 and the locking pin 18 is rotated from an unlocked position as shownin FIGS. 24-25 to a locked position as shown in FIGS. 26-28. Rotation ofthe locking pin 18 is terminated when the blocking surfaces 68 a, 68 bare adjacent to or abut the adjacent retention flanges 54 a, 54 b oruntil a click is heard or felt by the surgeon in step 112. In onevariation, the locking pin 18 is rotated 90 degrees from the unlockedposition to the locked position, however, the invention is not solimited and the locking pin 18 may be rotated anywhere betweenapproximately 30 degrees and 330 degrees into a locked position. As thelocking pin 18 rotates into the locked position, the camming surface 66deflects the retention flange 54 inwardly toward the center of theretaining ring 16. This inward deflection of the retention flange 54advantageously displaces any tissue or bone fragments that may interferewith the retention flange 54 effectively covering the bone screw 14.Furthermore, the inward deflection of the retention flange 54 by theblocking surface 68 results in the retention flange 54 contacting andcovering at least a portion of the screw head 40 exerting a force ontothe screw head 40. This feature advantageously prevents the bone screw14 from loosening before migrating back out of the through hole 40 andkeeps the screw 40 inside the through hole 40. Feedback is provided tothe surgeon in that the locking pin 18 cannot be moved into the lockedposition until and unless the screw head 40 has been fully inserted suchthat the screw head 40, in particular, the ledge 47 of the screw head 40is disposed distally of the retention flange 54. The feedback may alsobe visual, the result of the surgeon observing the position of thelocking pin 18 relative to the retention flange 54. The slit 62 may becolored to enhance visual feedback.

To remove the bone plate 12, the same instrument is used to rotate thelocking pin 18 from a locked position to an unlocked position in whichthe blocking surface 68 is not adjacent to or does not abut theretention flange 54 and the retention flange 54 is free to flexoutwardly with respect to the retention ring 16. Advantageously, sincethe instrument recess 46 on the screw head 40 is not blocked by theoverlaying retention flange 54, an instrument can be inserted into theinstrument recess 46 on the screw head 40 to remove the bone screw 14.Using the instrument to back out the screw 14 results in the screw head40 camming against the retention flange 54 deflecting it outwardly andout of the pathway of the screw 14 being removed. In another variation,an additional instrument may be employed to keep the retention flange 54flexed in the outward position while the bone screw 14 is backed out ofthe bone.

Although this application discloses certain embodiments and examples, itwill be understood by those skilled in the art that the presentinventions extend beyond the specifically disclosed embodiments to otheralternative embodiments and/or uses of the invention and obviousmodifications and equivalents thereof. Further, the various features ofthese inventions can be used alone, or in combination with otherfeatures of these inventions other than as expressly described above.Thus, it is intended that the scope of the present inventions hereindisclosed should not be limited by the particular disclosed embodimentsdescribed above.

We claim:
 1. A bone plate system, comprising: a bone screw having a headportion connected to a shank portion; a plate having at least onethrough-hole configured to receive the bone screw for attaching theplate to bone, the through-hole defining an entry and exit pathway forthe bone screw; a resiliently deflectable retention flange connected tothe plate and located in the pathway of the bone screw when in a normalundeflected position, wherein the bone screw is configured for insertioninto the through-hole past the retention flange such that at least aportion of the head portion is positioned distally of the retentionflange, the retention flange having a deflected position permittinginsertion of the bone screw into the through-hole; and a locking pinbeing movably connected with respect to the plate, the locking pinhaving at least one blocking surface, wherein the bone plate systemincludes a locked position in which the blocking surface of the lockingpin is in a position adjacent to the retention flange to prevent outwarddeflection of the retention flange and thereby maintain the retentionflange in the pathway of the through-hole and above the bone screw toprevent the bone screw from backing out of the through hole.
 2. The boneplate system of claim 1 wherein the bone plate system includes anunlocked position in which the retention flange is deflectable out ofthe pathway of the bone screw to permit passage of the bone screw intoor out of the through-hole.
 3. The bone plate system of claim 1 whereinthe locking pin further includes at least one camming surfaceinterconnected with the at least one blocking surface, the cammingsurface being configured to lead into the blocking surface.
 4. The boneplate system of claim 3 wherein the bone plate system includes anunlocked position in which the camming surface is located adjacent tothe retention flange and the retention flange is deflectable out of thepathway of the bone screw to permit passage of the bone screw into orout of the through-hole.
 5. The bone plate system of claim 1 whereinwhen in the locked position, the blocking surface is configured todeflect the retention flange inwardly toward a center of thethrough-hole.
 6. The bone plate system of claim 1 wherein the retentionflange is upwardly spaced from a surface of the plate and located abovethe through-hole in an undeflected position.
 7. The bone plate system ofclaim 1 wherein the retention flange is integrally formed with aretention ring configured to be disposed inside the at least onethrough-hole.
 8. The bone plate system of claim 1 wherein the retentionflange extends radially into an insertion pathway of the bone screw whenin a normal undeflected position.
 9. A bone plate system, comprising: abone screw having a head portion connected to a shank portion; a platehaving at least one through-hole configured to receive the bone screwfor attaching the plate to bone, the through-hole defining an entry andexit pathway for the bone screw; a resiliently deflectable retentionflange connected to the plate and located in the pathway of the bonescrew when in a normal undeflected position, wherein the bone screw isconfigured for insertion into the through-hole past the retention flangesuch that at least a portion of the head portion is positioned distallyof the retention flange, the retention flange being deflectable topermit insertion of the bone screw into the through-hole; and a lockingpin connected to the plate and mechanically coupled to at least oneadjacent retention flange, the locking pin having at least one blockingsurface and at least one camming surface, the locking pin being movableto selectively position the blocking surface and camming surfaceadjacent to the retention flange, wherein the blocking surface whenpositioned adjacent to the retention flange is configured to preventoutward deflection of the retention flange to prevent the bone screwfrom backing out of the through-hole, and the caroming surface whenpositioned adjacent to the retention flange is configured to allowclearance for the retention flange to deflect outwardly to permit thebone screw to pass into and out of the through-hole.
 10. The bone platesystem of claim 9 wherein the locking pin includes two blocking surfacesoppositely disposed from each other and two camming surfaces oppositelydisposed from each other and configured to block or cam two adjacentretention flanges simultaneously.
 11. The bone plate system of claim 9wherein the locking pin is configured to deflect the retention flangetoward a center of the through-hole.
 12. The bone plate system of claim9 wherein the locking pin includes a main body, the blocking surface andthe camming surface being formed along the perimeter of the main body,wherein the thickness of the main body along the perimeter in thelocation of the blocking surface is greater than the thickness of themain body along the perimeter in the location of the camming surface.13. The bone plate system of claim 9 wherein the locking pin defines acircular outer perimeter.
 14. The bone plate system of claim 9 whereinthe locking pin is configured such that the camming surface and theblocking surface in turn contact at least a portion of the retentionflange with movement of the locking pin relative to the plate.
 15. Abone plate system, comprising: a bone screw having a head portionconnected to a shank portion; a plate having at least one through-holeconfigured to receive the bone screw for attaching the plate to bone,the through-hole defining an entry and exit pathway for the bone screw;a retention member disposable in the through-hole and including acentral aperture having an entry opening at a top surface and adeflectable retention flange upwardly spaced from the top surface, thecentral aperture adapted to receive the bone screw, wherein the bonescrew is configured for insertion into the through-hole past theretention flange such that at least a portion of the head portion ispositioned distally of the retention flange; and a locking pin beingmovably connected with respect to the plate, the locking pin having atleast one blocking surface, wherein the bone plate system includes alocked position in which the blocking surface of the locking pin is in aposition adjacent to the retention flange to prevent outward deflectionof the retention flange and thereby maintain the retention flange in thepathway of the through-hole and above the bone screw to prevent the bonescrew from backing out of the through hole.
 16. The bone plate system ofclaim 15 wherein the retention flange extends radially into an insertionpathway of the bone screw when in a normal undeflected position.
 17. Thebone plate system of claim 15 wherein the retention flange is integrallyformed with a retention ring configured to be disposed inside the atleast one through-hole.
 18. The bone plate system of claim 17 wherein aneck connects the retention flange to the retention ring.
 19. The boneplate system of claim 17 wherein the retention ring defines the centralaperture adapted to receive the bone screw, the central aperture beingsmaller than the head portion of the bone screw.
 20. The bone platesystem of claim 15 wherein when in the locked position, the blockingsurface is configured to deflect the retention flange inwardly toward acenter of the through-hole.
 21. A bone plate system, comprising: a firstbone screw having a head portion connected to a shank portion; a platehaving a first through-hole sized to receive the first bone screw forattaching the plate to bone, the first through-hole defining an entryand exit pathway for the first bone screw; a first retention memberadapted for seating in the first through-hole, the first retentionmember having a retention flange and an aperture sized to allow passageof the shank but not the head portion of the first bone screw, theretention flange being upwardly spaced from the aperture of the firstretention member, wherein the head portion of the first bone screw ismovable past the retention flange so that the retention flange overliesat least a section of the head portion; and a locking pin being movablyconnected with respect to the plate, the locking pin having a firstblocking surface, wherein the bone plate system includes a lockedposition in which the first blocking surface of the locking pin ispositionable adjacent to the retention flange to prevent outwarddeflection of the retention flange and thereby maintain the retentionflange in an overlying position above the first bone screw to preventthe first bone screw from backing out of the first through-hole.
 22. Thebone plate system of claim 21 wherein the retention flange of the firstretention member is integrally formed with a retention ring configuredto be disposed inside the first through-hole.
 23. The bone plate systemof claim 22 wherein a neck connects the retention flange to theretention ring of the first retention member.
 24. The bone plate systemof claim 22 wherein the retention ring of the first retention memberincludes the aperture sized to permit passage of the shank portion ofthe first bone screw, but not the head portion.
 25. The bone platesystem of claim 21 further comprising: a second bone screw having a headportion connected to a shank portion; a second through-hole sized toreceive the second bone screw for attaching the plate to bone, thesecond through-hole defining an entry and exit pathway for the secondbone screw; a second retention member adapted for seating in the secondthrough-hole, the second retention member having a retention flange andan aperture sized to allow passage of the shank but not the head portionof the second bone screw, the retention flange being upwardly spacedfrom the aperture of the second retention member, wherein the headportion of the second bone screw is movable past the retention flange sothat the retention flange overlies at least a section of the headportion, and wherein the locking pin includes a second blocking surfaceand the locked position of the bone plate system is one in which thesecond blocking surface of the locking pin is positionable adjacent tothe retention flange of the second retention member to prevent outwarddeflection of the retention flange and thereby maintain the retentionflange in an overlying position above the second bone screw to preventthe second bone screw from backing out of the second through-hole. 26.The bone plate system of claim 21 wherein the retention flange of thefirst retention member is resiliently deflectable and extends radiallyinto the pathway of the first bone screw when in a normal undeflectedposition.
 27. The bone plate system of claim 21 wherein when in thelocked position, the first blocking surface is configured to deflect theretention flange inwardly toward a center of the first through-hole.